Hydrometallurgical treatment of metal-bearing materials, such as metal ores, metal-bearing concentrates, and other metal-bearing substances, has been well established for many years. Moreover, leaching of metal-bearing materials is a fundamental process utilized to extract metals from metal-bearing materials. In general, the first step in this process is contacting the metal-bearing material with an aqueous solution containing a leaching agent or agents which extracts the metal or metals from the metal-bearing material into solution. For example, in copper leaching operations, especially copper from copper minerals, such as chalcopyrite, chalcocite, covellite, malachite, pseudomalachite, azurite, chrysocolla, and cuprite, sulfuric acid in an aqueous solution is contacted with copper-bearing ore. During the leaching process, acid in the leach solution may be consumed and various soluble components are dissolved thereby increasing the metal content of the aqueous solution. Other ions, such as iron may participate in the leaching of various minerals as these ions participate in dissolution reactions.
The aqueous leach solution containing the leached metal can then be treated via a known process referred to as solution extraction wherein the aqueous leach solution is contacted with an organic solution comprising a metal-specific extraction reagent, for example, an aldoxime and/or ketoxime or a mixture thereof. The metal-specific extraction reagent extracts the metal  from the aqueous phase into the organic phase. Moreover, during the solution extraction process for copper and certain other metals, a leaching agent may be regenerated in the aqueous phase. In the case where sulfuric acid is the leaching agent, sulfuric acid is regenerated in the aqueous phase when copper is extracted into the organic phase by the extraction reagent. Iron ions, which should not be extracted by the metal-specific extraction reagent, should be recycled to the leaching step to the maximum extent possible.
In a standard agitation leaching process for copper, followed by solution extraction, the leach solution is diluted to a lesser or greater extent with acidified water in conjunction with the solid-liquid separation process needed to provide a clarified leach liquor and solid discharge. The diluted clarified leach solution then undergoes solution extraction wherein copper is removed from, and the sulfuric acid concentration is increased in, the aqueous phase. A portion of this copper-depleted, acid-containing aqueous phase, now called the raffinate, may be recycled back to the leaching process, recycled to the front of the solid-liquid separation process, and/or forwarded to secondary metal extraction processes, including but not limited to cobalt recovery.
However, under these current leaching and solution extraction processes, large concentrations of soluble metal and metal precipitate can be lost in the metal-depleted, acid-containing aqueous phase raffinate solutions. These losses lead to inefficiencies and low overall process yields. Additionally, these high metal concentrations in the raffinate make recovery of secondary metals costly and possibly impractical.
Accordingly, a process circuit for controlling the concentration of metal, especially copper, in the raffinate solution which is the feed for the subsequent recovery of secondary metals without negatively affecting the primary metal recovery circuit would be advantageous.